Sintered ceramic components are used instead of metallic parts in applications where properties such as high hardness, excellent high temperature behaviour, high thermal shock resistance, low density and high electrical insulating capacity are desired.
Some applications put extreme demands on the components, for example rolling bearings in which contacting parts, between rolling elements and rings, are exposed to high and local stresses. The contacting parts are also exposed to various lubrication, contamination and temperature conditions. Reliable performances and long operating life is critical. So called hybrid bearings consist of ceramic rolling elements and steel based rings with steel, plastic or brass-based cages. Hybrid bearings have shown benefits in certain operation conditions, such as high rotation speed, starved or poor lubrication conditions, high solid contamination conditions and conditions where electric current is passing through the bearings. A combination of high relative density, hardness, fracture toughness and strength appears necessary in order to design ceramic rolling elements for such applications. The mechanical properties of ceramic materials can be very good, but they are greatly influenced by internal as well as surface defects, such as inclusions of foreign matter, pores, large grains and cracks, and these features will define their reliability.
A rolling bearing consists of rolling elements such as balls, cylindrical rollers, spherical rollers, taped rollers and needles, between an inner ring and an outer ring.
The raw material for production of ceramic components is a powder produced through milling of the constituents in a liquid and subsequent drying of the slurry. The drying can be performed through, e.g., spray drying or freeze drying. It is common that the powder for bearing components is silicon nitride based. Small amounts of rare earth powder can be added for liquid phase sintering in the case of silicon nitride and sialons (silicon nitride based material where silicon and nitrogen are substituted with aluminium and oxygen). For formation of ceramic components in conventional processes, organic additives are often used as binders for forming green bodies by dry pressing or cold isostatic pressing. The organic additives need to be removed by burning the ceramic green body before final densification. For large ceramic components this process can take a long time and is more difficult for large bodies than for small or medium size components.
Several grades of ceramics for bearing parts have been developed and are available on the market. There is an increasing demand for large ceramic bearing components of good quality for various bearing applications. Silicon nitride and sialon are light, hard and strong engineering ceramics.
Silicon nitride and sialon bearing components are today primarily produced through hot isostatic pressing (HIP) or gas pressure sintering (GPS), methods where high sintering temperatures are combined with high pressure.
Manufacturing larger ceramic components of good quality for bearing applications tends to present challenges for pressing of the green bodies and the sintering process results in a costly product. Using cheaper raw materials and lower pressures during the sintering tend to yield insufficient or at least deteriorated mechanical properties which introduce limitations in the reliability and performance for demanding bearing operations.
The HIP process requires complicated capsules for enclosing the powder to be pressed and the pressing cycle is slow and has high energy consumption. The total production cycle for making large ceramic components using HIP is therefore several hours for each batch. A normal batch time cycle for a HIP produced component is around 8 hours and increasing for larger objects, making sintering of larger components very time consuming.
In gas pressure sintering the powder is first compressed at conventional pressures in order to close all surface pores and thereby avoid gas penetration during the second step when the pressure is raised. Sintering is mainly performed during the second step with high pressure and the pressure is normally kept at a high level also during cooling. The method is time and energy consuming with cycle times of several hours. Increasing the size of the sintered components leads to an exponential increase of time and energy consumption.
Much of the development of ceramic bearings has specifically been addressing small ceramic bearing components used in e. g. computer hard disk drives or other applications where low vibration operation is of high importance. Sintering of larger ceramic components is difficult through conventional methods. Inhomogeneous structures and difficulties with the densification are major problems as well as high costs, long production cycle times and high energy consumption.
Electric pulse assisted consolidation (EPAC) includes processes based on heating the material to be sintered with a pulsed DC current. Other names commonly used for this technique are spark plasma sintering (SPS), pulsed electric current sintering (PECS), field assisted sintering technique (FAST), plasma-assisted sintering (PAS) and plasma pressure compaction (P2C). These technologies will in this document hereafter be referred to as SPS. In SPS a uniaxial pressure is applied while the sample is being heated. The heating occurs through electrical energy pulses that are applied to the powder which is placed in a die between graphite punches. This sintering method allows the production of dense materials, while applying high heating rates and short dwell times. The technique uses a combination of high current and low voltage. A pulsed DC current with typical pulse durations of a few ms and currents of 0.5-30 kA flows through the punches, die and, depending on the electrical properties of the specimen, also through the specimen. The electrical pulses are generated in the form of pulse packages where the on: off relation is in the region of 1:99 to 99:1, typically 12:2 (12 pulses on, 2 off). The pressure is applied on the punches in a uniaxial direction, and is normally between 5 and 250 MPa.
U.S. Pat. No. 5,622,905 and U.S. Pat. No. 5,698,156 describe silicon nitride based bodies with uniform, fine crystal grains. The silicon nitride powder used should consist of grains smaller than 200 nm or grains having an amorphous structure. The ultrafine powder is prepared by chemical vapour deposition. The sintering temperature for silicon nitride powder is between 1200 to 1400° C. or between 1400 to 1900° C. or even higher temperatures if the total time times temperature is below 600000° C. sec. The SPS process can be used for manufacturing the bodies. A problem with this method is that it requires an expensive treatment, like chemical vapour deposition of the starting powder in order to ensure a fine grain size of the resulting product. It will also be difficult to use this method for larger components since that will require longer dwelling times which will not be compatible with the very fine grain structure.
U.S. Pat. No. 5,720,917 describes a method for making cheaper high quality silicon nitride products by adapting the manufacturing process so that less pure silicon nitride starting material can be used. The silicon nitride and sintering aids can have a metal impurity level up to 5000 ppm and can be treated at a sintering temperature of 1300 to 1900° C. and the product of sintering temperature and sintering time shall be between 105 to 106° C. sec.
U.S. Pat. No. 6,844,282 and U.S. Pat. No. 7,008,893 describe silicon nitride based sintered bodies with a particle size below 100 nm and very low friction coefficient. The method of production comprises pulverizing and mixing a silicon nitride powder, a sintering aid a metallic titanium powder and a graphite/carbon powder to a very small particle size, below 30 nm. The powder compact shall be sintered in a nitrogen atmosphere with a pressure between 0.05 to 1 MPa.
The Chinese patent CN1793042 presents spark plasma sintered silicon nitride with high rigidity and high tenacity. The silicon nitride comprises additives of rare earth oxides, alumina and aluminium nitride.
There is an increased demand for ceramic alternatives to metallic bearings for applications that require larger size components. Bearings are being used in applications where properties such as low weight, excellent mechanical properties and electrical insulation have high priority. Some of the properties of the ceramic bearings are also favourable for more environmentally friendly solutions, as for example a lower bearing weight may lead to reduced energy consumption. In order to use ceramic bearings on a larger scale the quality of the larger ceramic bearings has to be improved as well as the cost for producing these bearings must come down.